Conventionally, as a fastener element used in a slide fastener, there have been known synthetic resin fastener elements, each being individually formed by performing injection molding of synthetic resin on a fastener tape, continuous fastener elements formed by forming monofilament in a coil shape or a zigzag shape, metal fastener elements formed by swaging a Y-shaped metal element material onto a fastener tape, or the like.
The synthetic resin fastener elements are usually formed to straddle a fastener so as to be disposed on a first surface as an outer surface of the fastener tape and a second surface as a tape back surface. The synthetic resin fastener elements include an upper half element portion arranged on the first surface side of the fastener tape and a lower half element portion arranged on the second surface side of the fastener tape.
The synthetic resin fastener element is mostly formed such that the upper half element portion and the lower half element portion have symmetrical shapes, but there is a case in which the upper half element portion and the lower half element portion are asymmetrically formed in different shapes, for example, in order to improve the appearance, the feel (the touch), or the like of the slide fastener.
Examples of the fastener elements in which the upper half element portion and the lower half element portion have shapes different from each other are disclosed, for example, in Japanese Utility Model Application Publication No. 45-33956 (Patent Document 1), Japanese Patent Application Publication No. 47-37061 (Patent Document 2), Japanese Patent Application Laid-Open No. 2006-320642 (Patent document 3), and the like.
For example, a synthetic resin fastener element 51 described in Patent Document 1 includes an upper half element portion 53 arranged on a first surface side of a fastener tape 52 and a lower half element portion 54 arranged on a second surface side of the fastener tape 52 as illustrated in FIG. 12. The upper half element portion 53 includes a first tape-sandwiching portion 53a having a nearly rectangular shape in the front view and a triangular head 53b that extends from the first tape-sandwiching portion 53a toward the outside of the tape and is formed in a triangular shape in which a dimension in a tape length direction (hereinafter, this dimension is referred to as an element width dimension) gradually decreases toward the forefront thereof.
The lower half element portion 54 of the fastener element 51 includes a second tape-sandwiching portion that sandwiches the fastener tape 52 together with the first tape-sandwiching portion 53a, a neck 54a that extends from the second tape-sandwiching portion toward the outside of the tape and has a shape that is constricted in the element width direction, and a coupling head 54b that extends from the forefront of the neck 54a to be shaped like a bulge.
Particularly, in the fastener element 51 of Patent Document 1, an upper surface of the upper half element portion 53 and a lower surface of the lower half element portion 54 are continuous flat surfaces, and the triangular head 53b of the upper half element portion 53 is set to be thinner in thickness (dimension of the vertical direction) than the coupling head 54b of the lower half element portion 54.
According to Patent Document 1, since a slide fastener 50 is configured by using the above-described fastener element 51, the entire thickness of the fastener element 51 can be reduced. Further, since part of the coupling head 54b of the other coupling party can be covered with a side edge portion of the triangular head 53b, strong coupling to the extent that chain breaking is difficult to occur is obtained.
Next, a synthetic resin fastener element 61 described in Patent Document 2 includes an upper half element portion 62 and a lower half element portion 63 as illustrated in FIGS. 13 and 14. The upper half element portion 62 of each fastener element 61 has a nearly trapezoidal shape when viewed from the front and includes a first tape-sandwiching portion 62a that sandwiches a fastener tape together with the lower half element portion 63, a neck 62b that extends from the first tape-sandwiching portion 62a toward the outside of the tape, and a coupling head 62c that extends from a forefront of the neck 62b to form a bulge shape. In Patent Document 2, the coupling head 62c of the upper half element portion 62 is formed to have a cross section of a semicircular shape.
Meanwhile, the lower half element portion 63 of the fastener element 61 includes only a second tape-sandwiching portion 63a arranged to correspond to the first tape-sandwiching portion 62a and the neck 62b of the upper half element portion 62. Below the coupling head 62c of the upper half element portion 62, a space with nothing formed is present.
In the slide fastener 60 of Patent Document 2 having the above-described fastener element 61, since the coupling head 62c of the upper half element portion 62 has a semicircular cross section, the fastener tape 64 can be easily bent toward the upper half element portion 62 side at a small curvature in a state in which the left and right fastener elements 61 are coupled. According to Patent Document 2, a tape inner side edge portion of the first tape-sandwiching portion 62a in each fastener element 61 is formed to have a larger width than the coupling head 62c, and thus coupling of the left and right fastener elements 61 can be prevented from coming loose in the bending state of the fastener tape 64.
A synthetic resin fastener element 71 described in Patent Document 3 has an upper half element portion 72 and a lower half element portion 73 as illustrated in FIG. 15. The upper half element portion 72 of each fastener element 71 includes a first tape-sandwiching portion 72a having a nearly rectangular shape when viewed from the front and a triangular head 72b that extends from the first tape-sandwiching portion 72a toward the outside of the tape and is formed in a triangular shape in which an element width dimension gradually decreases toward the forefront.
The lower half element portion 73 of the fastener element 71 includes a second tape-sandwiching portion 73a that sandwiches a fastener tape 74 together with the first tape-sandwiching portion 72a, a neck 73b that extends from the second tape-sandwiching portion 73a toward the outside of the tape, and a coupling head 73c that extends from a forefront of the neck 73b to form a bulge shape.
Further, in the fastener element 71, an upper surface of the upper half element portion 72 is formed to be curved such that a central portion in an element width direction and a central portion in an element length direction can protrude upward. Meanwhile, a lower surface of the lower half element portion 73 is formed to be a flat plane surface.
In a slide fastener 70 having the fastener element 71 of Patent Document 3, the upper surface of the upper half element portion 72 has the curved surface that bulges in the form of a gentle circular arc, and thus the feel or the touch of the fastener element 71 can be improved, and the appearance of the element row can be improved.    Patent Document 1: Japanese Utility Model Application Publication No. 45-33956    Patent Document 2: Japanese Patent Application Publication No. 47-37061    Patent Document 3: Japanese Patent Application Laid-Open No. 2006-320642